Evidence for the health benefits of regular physical activity is clear and unanimous. Recently, observational studies have found that prolonged bouts of sitting time are associated with chronic disease and mortality, even in individuals who engage in regular physical activity. These new findings indicate that sedentary behaviour should be regarded as a distinct class of behaviour with independent effects on disease risk, separate from the behaviour of leisure time physical activity. Prolonged daily sitting time is prevalent in modern, western society, making further exploration into this field of research a relevant public health issue.